1. Field of the Invention
The present invention relates to a method and an apparatus for repeatedly erasing an ink-carrying layer from the surface of an image-containing printing form which is usable for offset printing.
2. Description of the Related Art
As proposed, for example, in German Patent 39 17 844 owned by the instant assignee, a direct image application, in this case by means of a thermo-transfer method , is conventionally carried out by applying on a hydrophilic aluminum plate organic substances which influence in accordance with the image the manner in which the ink is carried. The substances are applied on portions of the surface of the printing form by means of an image point transfer unit in accordance with a digital image information. For this purpose, a thermo-transfer foil is used which has on its side facing the printing form a thermo-sensitive coating. By applying energy, the oleophilic image elements are dissolved or removed from the coating, transferred to the printing form and are anchored as an ink-carrying layer on the surface of the printing form.
However, the ink-carrying layer can also be anchored on the surface of the printing form as an image by means of the ink-jet process or by means of an electrostatic process utilizing a toner.
In addition to the hydrophilic aluminum plate, it is also possible to use as the printing form a printing cylinder with a cylinder jacket of ceramic material, or a solid ceramic or glass cylinder can be used.
As is readily apparent in the use of printing cylinders, it should be possible to repeatedly use the printing forms to which the image is applied directly. For this purpose, the printing forms to which the image has been applied in the above-described manner must be regenerated, i.e., the material forming the printed areas must be removed or erased and it may additionally be necessary to subject the surface of the printing form to a treatment to render it hydrophilic.
Previously used methods of erasing the subject on the printing form, such as softening, vaporizing, decomposing, burning of the image elements, frequently have the disadvantage that the method is carried out in several stages which means that the method is cumbersome and the surface of the printing form is subjected to a high mechanical or abrasive load.
In addition, erasing of the surface of the printing form is usually carried out after the printing process. Thus, initially the inking device and the damping device are turned off and the remaining residues of the printing ink on the form cylinder are removed by means of cleaning devices which are conventionally used in machines of this type.
German application P 41 23 959.8 discloses a method of regenerating printing forms to which an image is applied directly and which are suitable for offset printing, in which the image on the surface of printing forms is erased and the surface is rendered hydrophilic without damaging or attacking the surface and which is comparatively simple.
The method described in the German application makes it possible to reactively erase organic substance particles or components on the surface of the printing form which had been applied thereon by means of ink-jet methods, electrostatic methods as well as thermo-transfer methods.